This invention relates to a holographic medium and a process for the use thereof to form a hologram. More specifically, this invention relates to such a medium containing a polymer which separates into two components during the holographic exposure.
In prior art processes for the formation of volume-phase holograms, interference fringes are formed within a holographic recording medium comprising a homogeneous mixture of a polymerizable monomer or oligomer and a polymeric binder; the polymerizable monomer or oligomer must of course be sensitive or sensitized to the radiation used to form the interference fringes. In the light areas of the fringes, the monomer or oligomer undergoes polymerization to form a polymer that has a refractive index different from that of the binder. Diffusion of the monomer or oligomer into the light areas, with consequent chemical segregation of binder from these areas and its concentration in the dark areas, produces spatial separation between the polymer formed from the monomer or oligomer and the binder, thereby providing the refractive index modulation needed to form the hologram. Typically, after the holographic exposure, a post-treatment step of blanket exposure of the medium to actinic radiation is required to complete the polymerization of the monomer or oligomer and fix the hologram.
There are two major difficulties in formulating this type of holographic recording medium. Firstly, the components of the medium must be compatible with one another so that the medium does not separate into two discrete phases before the holographic exposure, and for a practical commercial product, it is necessary that the recording medium not undergo phase separation during several months of storage during which the medium may be subjected to wide temperature variations, for example in trucks during delivery of the medium. However, it is also desirable that the two phases produced from the medium during the holographic exposure show a large difference in refractive index, since such a large difference in refractive index improves the holographic efficiency of the hologram produced. Unfortunately, these requirements of compatibility and high refractive index difference tend to be inconsistent with one another. A high refractive index component will typically contain numerous aromatic nuclei, while a low refractive index component will typically contain fluorinated alkyl or similar groups, and such aromatic and fluorinated alkyl components tend not to form homogeneous mixtures. It is difficult to produce, for example, a recording medium, comprising a highly aromatic polymer and a perfluoroalkyl acrylate or methacrylate monomer or oligomer, which is homogeneous and stable for long periods over a wide temperature range. Although useful media can be formulated, the ranges of components which can be used are limited (which tends to increase the cost of the medium) and in practice workers are usually forced into somewhat unsatisfactory compromises, resulting in media which have workable (though less than optimum) compatibility and workable (though less than optimum) refractive index differences.
Secondly, the physical properties of such holographic recording media tend to be unsatisfactory. Since such media comprise a mixture of a polymeric binder and a polymerizable monomer or oligomer, they tend to be soft, or even semi-solid, with considerable tackiness. Such soft, tacky mixtures are difficult to handle without damage to the thin layer of the medium normally used to form a hologram, and are likely to pick up dirt, dust and other materials during storage and handling; the presence of dust, dirt etc. on the medium is of course highly undesirable since the quality of the resultant hologram is adversely affected.
The present invention provides a holographic recording medium which is essentially free from the problems of incompatibility and tackiness experienced in prior art media.
Accordingly, this invention provides a process for preparing a hologram. This process comprises:
providing a holographic recording medium comprising (a) an acid generator capable of generating an acid upon exposure to actinic radiation; and (b) a polymer, this polymer comprising a plurality of high refractive index moieties, a plurality of low refractive index moieties, and a plurality of acid-sensitive linking groups connecting the high and low refractive index moieties, the acid-sensitive linking groups being capable of being cleaved by the acid generated by the acid generator; and
passing into the medium a reference beam of coherent actinic radiation to which the acid generator is sensitive and an object beam of the same coherent actinic radiation, thereby forming within the medium an interference pattern and, in the light areas of this interference pattern, causing generation of acid by the acid generator and cleavage of the acid-sensitive linking groups by this acid, thereby causing separation of either the high or the low refractive index moieties from the polymer, and formation of a hologram within the medium.
This invention also provides a holographic recording medium comprising:
(a) an acid generator capable of generating an acid upon exposure to actinic radiation; and
(b) a polymer, this polymer comprising a plurality of high refractive index moieties, a plurality of low refractive index moieties, and a plurality of acid-sensitive linking groups connecting the high and low refractive index moieties, the acid-sensitive linking groups being capable of being cleaved by the acid generated by the acid generator, thereby causing separation of either the high or the low refractive index moieties from the polymer, the medium being capable, upon exposure to an interference pattern formed between two beams of the actinic radiation, of forming regions differing in refractive index and thereby producing a hologram.